Delta-sigma modulators are particularly useful in digital-to-analog and analog-to-digital converters (DACs and ADCs). Using oversampling, the delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta-sigma modulator performs noise shaping by acting as a low-pass filter to the input signal and a high-pass filter to the noise. Most of the quantization noise power is thereby shifted out of the signal band.
Delta-sigma modulators can be designed from various configurations, including the switched-capacitor approach and the continuous-time approach. Continuous-time delta-sigma modulators have certain advantages over switched-capacitor designs, such as certain designs having protection against signal aliasing and the possibility of wideband converters. Some conventional continuous-time delta-sigma modulators are shown in FIG. 1 and FIG. 2.
FIG. 1 is a circuit schematic illustrating a continuous-time delta-sigma modulator with a feedforward path. Delta-sigma modulator 100 receives an analog input signal and provides a digital output signal representative of the input analog signal. The delta-sigma modulator 100 has a feedforward path that includes resistors R, capacitors C, a first integrator 112, a second integrator 114, a third integrator 116, a summer 102, and a quantizer 104 coupled in series between the input and the output. The delta-sigma modulator 100 also has a feedback path that comprises a delay block 106, a digital-to-analog converter (DAC) block 108, and a resistor R coupled in series between the quantizer 104 and the first integrator 112.
The delta-sigma modulator 100 is conventionally referred to as a feedforward modulator, as the noise-transfer function (NTF) characteristics are set by the coefficients c0, c1, c2, and c3 fed forward to the summer 102. The three coefficients c1, c2, c3 allow for setting the poles of any noise transfer function (NTF). Feedforward term c0 from the input signal may be adjusted to obtain flexibility in setting the signal-transfer function (STF).
Advantages to feedforward delta-sigma modulators, such as shown in FIG. 1, include no signal energy provided at the outputs of the integrators 112, 114, and 116, which allows for better linearity. Another advantage is that only one DAC output signal, such as from DAC 108, is required or needs to be processed. Some disadvantages are that such feedforward delta-sigma modulators require a summer block, such as summer 102, and that the first integrator 112 must be low noise and fast. Another disadvantage is that feedforward delta-sigma modulators provide minimal natural anti-aliasing.
Another configuration for a continuous-time delta-sigma modulator implements feedback into the integrators of the filter. FIG. 2 is a circuit schematic illustrating a continuous-time delta-sigma modulator with multiple feedback paths. Delta-sigma modulator 200 receives an analog input signal and provides a digital output signal representative of the input analog signal. Delta-sigma modulator 200 has a feedforward path that includes resistors R, capacitors C, a first integrator 212, a second integrator 214, a third integrator 216, and a quantizer 204 coupled in series between an input and an output. The delta-sigma modulator 200 also includes a feedback path that comprises a delay block 206, a digital-to-analog converter (DAC) block 208, and resistors R1, R2, and R3 coupled between the output and each of the integrators 212, 214, and 216. Resistors R1, R2, and R3 are coupled to the inputs of respective integrators 212, 214, and 216.
The characteristics of the delta-sigma modulator 200 may be determined by setting values for resistors R1, R2, and R3. These values set the NTF coefficients, similar to the values of coefficients c1, c2, and c3 of FIG. 1. Advantages of the feedback-type delta-sigma modulator of FIG. 2 are that no summer is required, that some reduced speed constraints exist for the first integrator 212, and that the low-pass signal-transfer function (LP STF) provides inherent anti-aliasing characteristics. Disadvantages of the feedback-type delta-sigma modulator of FIG. 2 are that full signal swing exists at the outputs of all integrators 212, 214, and 216 placing stricter requirements on the integrators 212, 214, and 216, and that the DAC block 208 has to drive and feedback multiple outputs to the integrators 212, 214, and 216, which increases the complexity of the circuit.
A number of other modulator structures that incorporate both feedback and feedforward techniques exist, but generally trade one characteristic for another. These mixed modulator structures allow for various STF numerators with the denominator of the STF being shared with the NTF. Local feedback between the integrators can also be used to select the zero locations of the NTF, which in one configuration could set all of the NTF zeroes at Z=−1 or at a Direct Current (DC) value.
One example configuration showing a common challenge with these conventional modulator structures is provided by U.S. Pat. No. 7,375,666 entitled “Feedback Topology Delta-Sigma Modulator having an AC-coupled Feedback Path” to Melanson (hereafter the '666 patent), which is hereby incorporated by reference. The '666 patent shows an example feedback-type delta-sigma modulator. The '666 patent includes descriptions of modulator structures that remove the signal component from the output of an integrator in the filter. The first integrator of the modulator must have a relatively fast response, because it receives a feedback signal with fast edges which requires a high speed design in order to keep non-linear operation from happening in the first integrator. Additionally, certain capacitors of the modulator structure of the '666 patent need to be relatively large. These large capacitors increase the size and cost of an integrated circuit (IC) that encompasses such a feedback delta-sigma modulator.
The demands on the first integrator for a delta-sigma modulator can be minimized by using more bits in the DAC feedback element, but practical size and cost considerations limit how far that can be pushed. Thus, the conventional modulator structures used, for example, in analog-to-digital converters (ADC) fail to adequately process high-frequency feedback signals.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for analog-to-digital converters, such as those that may be employed in consumer-level devices, such as mobile phones, or other commercial technology employed in electronic detectors. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art.